A technique commonly employed in the art for tempering glass sheets, especially when the sheets are to be bent and then tempered, is to vertically hang each glass sheet from tongs which grip the upper marginal edge portion of the glass sheet, whereby the glass sheet may be conveyed while thus suspended through heating, bending, and tempeing steps. Tongs of this type are disclosed in U.S. Pat. No. 3,089,727 to W. J. Hay. The heating step entails suspending the glass sheets within a heating chamber until the temperature of the glass approaches its softening point, and then each sheet in series is conveyed out of the heating chamber and (optionally) into a bending station. A typical vertical bending operation is disclosed in U.S. Pat. No. 3,367,764 to S. L. Seymour, wherein a heat-softened glass sheet is bent by being pressed between a pair of complementary, horizontally reciprocated bending molds. After bending, with the glass sheet still at an elevated temperature, the sheet is conveyed into a tempering station where it is rapidly cooled by blasts of tempering medium so as to establish compressive stresses in the surface portions of the sheet, thereby strengthening the sheet. The tempering medium is usually air, but as used herein, the term may encompass any fluid capable of cooling a hot glass sheet. Such a process has proved to be an economical, high speed method for mass-producing tempered glass sheets, especially bent and tempered automotive glazing and the like.
One difficulty encountered with tempering glass sheets freely hung from tongs is that directing blasts of tempering medium onto the glass sheets can sometimes cause the glass sheets to sway from side to side. Although measures are ordinarily taken to equalize the force of the impingement on both sides of the glass sheet, it is usually found to be virtually impossible to precisely duplicate flow conditions on both sides of a glass sheet, particularly if it is bent, and as a result, sharp side-to-side oscillations of the glass sheet and the tongs are induced. This buffeting of the glass sheets impedes uniform application of the tempering medium onto the glass sheets, which in turn leads to imbalanced stresses in the tempered product. Such uneven stresses can result in the tempered glass sheet failing to meet strength specifications and may even cause glass breakage during processing. The problem of buffeting is especially troublesome with thin glass (i.e., glass about 4 millimeters or less in thickness), the demand for which has been increasing for use in automobiles. Not only is thin glass lighter in weight and thus more susceptible to buffeting, but also the need for faster rates of cooling to temper thin glass entails the use of higher pressure blasts of tempering medium, which additionally increases the amount of buffeting. Efforts to reduce buffeting in the prior art have included the use of guide wires extending through a quenching apparatus as shown in U.S. Pat. No. 4,006,002 to Hetman, but such an approach has not been found adequate to stabilize glass sheets in the quench to the extent desired.